Magnetic memories, particularly magnetic random access memories (MRAMs), have drawn increasing interest due to their potential for high read/write speed, excellent endurance, non-volatility and low power consumption during operation. An MRAM can store information utilizing magnetic materials as an information recording medium. One type of MRAM is a spin transfer torque random access memory (STT-RAM). STT-RAM utilizes magnetic junctions written at least in part by a current driven through the magnetic junction. A spin polarized current driven through the magnetic junction exerts a spin torque on the magnetic moments in the magnetic junction. As a result, layer(s) having magnetic moments that are responsive to the spin torque may be switched to a desired state.
For example, FIG. 1 depicts a conventional dual magnetic tunneling junction (MTJ) 10 as it may be used in a conventional STT-RAM. The conventional dual MTJ 10 typically includes a first conventional pinned layer 12, a first conventional tunneling barrier layer 14, a conventional free layer 16, a second conventional tunneling barrier 18, and a second conventional pinned layer 20. The conventional tunneling barrier layers 14 and 18 are nonmagnetic and are typically a thin insulator such as MgO.
The conventional pinned layers 12 and 20 and the conventional free layer 16 are magnetic. The magnetic moment 13 of the conventional pinned layer 12 is fixed, or pinned, in a particular direction. The magnetic moment 21 of the conventional pinned layer 20 is fixed, or pinned, in a particular direction, typically substantially opposite to the direction of the magnetic moment 13 of the conventional pinned layer 12. Although depicted as a simple (single) layer, the conventional pinned layers 12 and 20 may include multiple layers. For example, the conventional pinned layer 12 and/or 20 may be a synthetic antiferromagnet (SAF) including magnetic layers antiferromagnetically coupled through thin conductive layers, such as Ru. In such a SAF, multiple magnetic layers interleaved with a thin layer of Ru may be used. Alternatively, the coupling across the Ru layers can be ferromagnetic.
The conventional free layer 16 has a changeable magnetic moment 17. Although depicted as a simple layer, the conventional free layer 16 may also include multiple layers. For example, the conventional free layer 16 may be a synthetic layer including magnetic layers antiferromagnetically or ferromagnetically coupled through thin conductive layers, such as Ru. The pinned layers 12 and 20 and free layer 16 have their magnetic moments 13, 21, and 17, respectively, oriented perpendicular to the plane of the layers. In another devices, the magnetic moments 13, 21, and 17 can be substantially in the plane of the layers.
To switch the magnetic moment 17 of the conventional free layer 16, a current is driven perpendicular to plane (in the z-direction). When a sufficient current is driven from the conventional pinned layer 12 toward the pinned layer 20, the magnetic moment 17 of the conventional free layer 16 may switch to be parallel to the magnetic moment 21 of the conventional pinned layer 20. When a sufficient current is driven from the conventional pinned layer 20 toward the conventional pinned layer 12, the magnetic moment 17 of the free layer 16 may switch to be parallel to that of the pinned layer 12. The differences in magnetic configurations correspond to different magnetoresistance levels and thus different logical states (e.g. a logical “0” and a logical “1”) of the conventional MTJ 10.
Although the conventional MTJ 10 may be written using spin transfer and used in an STT-RAM, there are drawbacks. The conventional dual MTJ 10 is desired to be thermally stable. As such, Δ for the free layer 16, the thermal stability coefficient, may be desired to be high. However, a low switching current for spin transfer-based switching is also desired. In the macrospin approximation, the switching current is proportional to (α/η)Δ, where α is the magnetic damping coefficient, η is the spin torque efficiency, and Δ is the thermal stability coefficient described above. The switching current thus tends to increase with increases in the thermal stability coefficient. Stated differently, as the magnetic junction 10 becomes more thermally stable, the switching current increases.
Accordingly, what is needed is a method and system for improving the thermal stability of the magnetic junction without substantially increasing the switching current. The method and system described herein address such a need.